Organic light emitting devices (OLED's) are emissive displays consisting of a transparent substrate coated with a transparent conducting material, such as Indium Tin oxide (ITO), one or more organic layers and a cathode made by evaporating or sputtering a metal of low work function characteristics, such as Ca or Mg. The organic layers are chosen so as to provide charge injection and transport from both electrodes into the electroluminescent organic layer (EL) where the charges recombine, emitting light. There may be one or more organic hole transport layers (HTL) between the ITO and the EL, as well as one or more electron injection and transporting layers (EL) between the cathode and the EL.
To fabricate multicolor and full color displays, two approaches have been proposed. The first approach consists of depositing emitting materials with different spectral characteristics. Each of these materials (typically one material with red emission, one for green and one for blue) is deposited separately in different parts of the substrate to achieve "full color" (RGB) pixels by separately powering the three color pixels.
The second approach consists of depositing a single emissive material and using (RGB) filters, resonant cavities and/or photoluminescent materials that can absorb the light from the emissive material and re-emit light at longer wavelengths (green and red). This approach requires that the emissive material has a substantial emission in the blue. In one prior art device (U.S. Pat. No. 5,126,214), the wavelength converting layers are deposited on a glass substrate as rows of green and red dyes dispersed in polymer films. Rows of blue filtering material are also deposited on the glass to enhance the blue emission of the device. A planarization layer is then applied over the wavelength converting layers. The ITO electrodes are then patterned over the planarized layer. A blue emitting dye is deposited over the ITO. The remaining layers of the OLED are then deposited as described above.
Devices based on color conversion do not require that the electroluminescent characteristics of three different emissive materials be balanced. This is very difficult to achieve in practice because the voltage-current characteristics and quantum efficiencies are usually very different for each material. EL lifetimes are also quite difficult to equalize between different materials.
Unfortunately, the color conversion system has its own problems. To prevent color anomalies when the device is viewed at an angle different from the normal to the surface, the emissive material and the color conversion material must lie very close to each other. This is particularly difficult to achieve with pixels that are less than 0.5 mm because of the thickness of the planarization material and the lack of uniformity in height of the color conversion materials. In general, the thickness of the color conversion layer for converting blue light to red light is significantly greater than the thickness of the layer for converting the blue light to green light. For example, the thickness of color conversion layers disclosed in U.S. Pat. No. 5,126,214 ranges from 80 to 800 microns. Accordingly, there is a gap that is filled with the planarization material over the green conversion material. This gap places a limit on the distance between the light emissive material and the green and blue conversion materials.
The need to provide the planarization layer also increases the complexity and cost of fabrication. The planarization step increases the number of steps in the fabrication process, and hence, increases the cost of fabrication. Cost of fabrication is particularly important in OLED displays, since such displays are being explored as a lower cost alternative to other display technologies.
Finally, prior art color conversion systems do not lend themselves to the production of flexible displays. One advantage of OLED based displays is the promise of providing flexible displays for use in automobile control panels in which the display would be bonded to a curved surface to improve its visibility. The color conversion layers and subsequent planarization step require the use of rigid glass substrates which increase the cost and weight of the displays in addition to preventing the displays from flexing.
Broadly, it is the object of the present invention to provide an improved OLED display.
It is a further object of the present invention to provide an OLED display based on color conversion that does not require the planarization layer utilized in prior art displays.
It is a still further object of the present invention to provide an OLED display in which the distance between the emissive layer and the color conversion layers is constant for all of the color conversion materials.
It is yet another object of the present invention to provide a flexible OLED display based on color conversion.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.